1. Field of the Invention
The present invention relates to a proximity detector for detecting the proximity of a substrate by a change of electrostatic capacity.
2. Description of the Prior Arts
A proximity detector is disposed at the front edge of a door such as an elevator so as to detect an obstacle, in the way of movement of the door during the time of shutting the door or at the full opening of the door under a non-contact condition whereby the movement of the door is reversed or the operation of the movement of the door is stopped.
The conventional proximity detector has the structure shown in FIG. 1.
In FIG. 1, the reference numerals (1) and (2) respectively designate antennae disposed at the front side of the door and (3) and (4 respectively designate earth electrostatic capacities of the antennae (1), (2); (5) designates a shield plate which is insulated from the antennae (1), (2); (6) and (7) respectively designate electrostatic capacities between the antennae (1), (2) and the shield plates (5). The electrostatic capacities (3), (4), (6), (7) form an impedance bridge. The reference numeral (8) designates a AC power source; (9) designates a transformer for detecting the unbalance of the bridge; (10) designates an AC amplification circuit for amplifying the unbalanced voltage of the bridge applied to the transformer (9). The reference numeral (11) designates a rectifying circuit; (12) designates a DC amplification circuit for amplifying the output of the rectifying circuit (11); (13) designates an output relay; (14) designates an antenna cover for protecting the antennae (1), (2) disposed at the front side of the door and D designates a non-sensitive part.
Normally, the bridge circuit is balanced to zero the output of the transformer (9). When a body or a substrate approaches the antenna (1), only the earth electrostatic capacity (3) of the antenna (1) is changed whereby the impedance bridge becomes unbalanced and an output voltage which is proportional to the unbalanced value is produced in the transformer (9). The output is amplified by the AC amplification circuit (10) and it is converted to DC current by the rectifying circuit (11) and the output is amplified by the DC amplification circuit (12) to drive the relay (13). The relay (13) stops the movement of shutting the door or reverses movement of the door so as to prevent the catching of a body or a substrate in the door.
In the conventional proximity detector having the bridge structure, changes in the electrostatic capacities in the same phase mode for both of the electrostatic capacities (3), (4) of the antennas (1), (2) cancel each other in the bridge circuit, whereby the erroneous operation caused by the swinging of the door during the operation of shutting the door can be eliminated and high detecting sensitivity can be attained. On the other hand, when one's hand is caused to touch the antenna cover (14) at the full opening condition of the door to prevent the initiation of the shutting operation of the door, the following disadvantages are caused.
(a) When one's hand is caused to touch the antenna cover (14) at the nonsensitive zone D in the boundary between the antenna (1) and the antenna (2), the bridge is balanced so as not to generate the bridge output whereby the shutting operation of the door cannot be prevented.
(b) When the hand of one person is caused to touch the antenna cover (14) over the antenna (1), and a hand of another person is caused to touch the antenna cover (14) over the antenna (2), the bridge is balanced whereby the shutting operation of the door cannot be prevented.
In order to eliminate the non-sensitive zone D causing the above-mentioned trouble, it can be considered that the antennas (1), (2) are further divided to form a plurality of bridge circuits by combining the divided antennae. However, the structure of such a device is complicated.
It is difficult to overcome the disadvantages of (2) by the modification of the bridge structure in principle.